The present invention relates to a load sensor and, in particular, to a load sensor for measuring belt tension in dynamic systems such as an idler or torque transmitting pulley.
In vehicle engines which have a number of belt driven components, proper belt tension is important to reducing belt noise, increasing belt life, and enhancing performance. If the belt is too tense, the belt""s effective life will be reduced and increased belt noises will occur during vehicle operation. If the belt is too slack, slippage between the belt and its associated pulleys may occur, thereby causing a deleterious effect on engine performance.
A number of devices are known for measuring belt tension. Three-point tension measurement devices are crude devices which are not often used because of two major drawbacks. First, the device is bulky and cannot be used with tight belt drives, such as timing belt drives in automotive engines. Second, the device considerably changes the dynamic behavior of the belt system, and therefore does not provide accurate measurements.
Static and dynamic belt span vibratory frequency measurement devices, such as clavis gauges or laser probes, measure the frequency of a laterally vibrating belt. The measurement can thereafter be used to calculate the actual belt tension, providing the mass of the belt and the span end pivot conditions are known. Because these conditions are not always constant, this method is not always accurate. These inaccuracy problems increase when measuring the frequency of a running belt, especially during mixed mode belt vibrations. Furthermore, measuring the frequency of a running belt only reveals the average dynamic tension, not the highs and lows of the belt tension.
Belt tension can also be measured using tension-sensitive coatings on the belt. This method, however, is highly sensitive to other belt stresses (e.g., belt twisting) in addition to pure tension. Also, this method is cumbersome, expensive, and unreliable in environmentally demanding conditions such as automotive engines.
Torque sensors on driven and driving pulleys or sprockets are commercially available devices that measure belt tension with relatively high accuracy. These sensors, however, can seldom be used due to space limitations, especially on timing belt drives. Furthermore, their high inertia makes them unacceptable for measuring dynamic system behavior.
Custom-made strain gauged drive components may also be used to measure belt tension, but these are time consuming and expensive to manufacture because of their customized nature. In addition, these devices are generally inaccurate due to lack of thermal compensating. Furthermore, in most cases, the strain gauged section of the device is relatively far from the belt/pulley interface, introducing errors, especially those caused by inertia in high frequency measurement conditions. This usually results in the device itself vibrating, which can be witnessed as negative force readings, noise, and high hysteresis value readings. Finally, these types of devices are highly sensitive to belt mistracking, i.e., belt centerline variation.
It is therefore an object of the present invention to provide a pulley assembly in which dynamic belt tension can be accurately measuring without affecting the dynamic behavior of the driven system. In order to achieve this object, the present invention provides a pulley assembly for measuring driving element tension in a system driven by a tensioned endless driving element. The pulley assembly comprises a rotatable pulley member having a driving element engaging outer surface engageable with the tensioned driving element such that the driving element applies a load to the pulley member directly related to the driving element tension. The load has a force component in a load measuring direction. The pulley member is mounted to a shaft assembly also comprises a load sensor which in turn comprises an annular gauge ring comprising an annular inner ring portion and an annular outer ring portion. The gauge ring is operatively associated with one of the pulley member and the shaft such that the load applied to the pulley member causes relative movement between the ring portions. The ring has strainable beam members extending in the load measuring direction and flexing beam members extending in a transverse direction generally perpendicular to the load measuring direction. The strainable beam members and the flexing beam members interconnect the ring portions.
The flexing beam members are thicker in comparison to the strainable beam members so that the flexing beam members are less subject to elongation and compression due to strain in comparison to the strainable beam members such that the flexing beam members substantially limit the relative movement between the ring portions to the load measuring direction when the load is applied to the pulley member by (1) resisting elongation to substantially prevent relative movement between the ring portions in the transverse direction and (2) flexing to allow limited relative movement between the ring portions in the load measuring direction. The strainable beam members are constructed and arranged such that the limited relative movement between the ring portions creates a strain in the load measuring direction in the strainable beam members. The strain has a magnitude directly related to a magnitude of the force component in the load measuring direction. The load sensor also comprises a strain measuring and outputting device operable to measure the magnitude of the strain and to thereafter output the measured strain magnitude as an output signal which can be used to calculate the force component magnitude and hence the driving element tension.
The pulley assembly of the present invention has a number of advantages over tension measuring devices known heretofore. Most importantly, the use of the load sensor in the pulley assembly of the present invention does not significantly affect the dynamic behavior of the driven system. Therefore, it is possible to obtain accurate readings of the belt tension as they would be found in practical applications. In addition, because the strainable beam members are relatively thin they are sensitive to the applied load and the resulting measurements are not affected by any transverse loading components because of the relative thickness of the transverse flexing beam members. Furthermore, the load sensor in the pulley assembly of the present invention can be arranged in close proximity to the belt/pulley interface such that slight variations in belt tension can be sensed by the measuring and outputting device. Thus, the load sensor of the present invention provides enhanced sensitivity to dynamic load changes.
The pulley assembly of the present invention can take a variety of forms. As will be seen from the following detailed description and the accompanying drawings, the gauge ring may be fixedly mounted to a fixed shaft with the pulley member rotatably mounted to the outside of the gauge ring, preferably by a ball bearing assembly. Additionally, the shaft may be rotatable and the gauge ring may be fixedly mounted to the shaft with the pulley member fixedly mounted to the gauge ring such that all three components rotate together. This arrangement can be particularly useful not only in an idler pulley assembly but also in a torque transmitting pulley assembly.
In its broadest aspects, the present invention is concerned with the load sensor itself out of the pulley assembly environment. The load sensor of the present invention comprises an annular gauge ring comprising an annular inner ring portion and an annular outer ring portion. The gauge ring has strainable beam members and flexing beam members. The beam members interconnect the ring portions. The flexing beam members are thicker in comparison to the strainable beam members so that the flexible beam members are less subject to elongation and compression due to strain in comparison to the strainable beam members such that the flexing beam members substantially limit the relative movement between the ring portions to the load measuring direction when a load with a force component in the load measuring direction is applied to one of the ring portions by (1) resisting elongation and compression to substantially prevent relative movement between the ring portions in a transverse direction and (2) flexing to allow limited relative movement between the ring portions in the load measuring direction. The strainable beam members are constructed and arranged such that the substantially isolated relative movement between the ring portions creates a strain in the load measuring direction in the strainable beam members. The strain has a magnitude directly related to a magnitude of the force component in the load measuring direction. A strain measuring an outputting device is adapted to measure the magnitude of the strain and to thereafter output the measured strain magnitude as an output signal which can be used to calculate the force component magnitude and hence the applied load.
The load sensor itself can be used in a variety of applications. As can be appreciated from the above discussion and the following detailed description, the load sensor of the present invention has many commercially advantageous applications for measuring belt tension in systems driven by an endless belt. The load sensor of the present invention may also be used to measure friction between an oscillating shaft and a bushing as disclosed hereinbelow. It is to be understood that the load sensor of the present invention may be applied to a wide variety of measuring applications and not only to those specifically disclosed in the present application.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.